Luteinizing hormone/choriogonadotropin receptor (LHR) plays a central role in human male sexual development. Mutation of the LHR results in abnormal production of testosterone and diseases of sexual development. Though the role of LHR in transducing the signal of luteinizing hormone/chorionic gonadotropin is well established, the mechanism of its action is still not fully understood. We had studied the molecular genetics of the LHR of a large number of patients with activating and inactivating mutations of the LHR. These studies help us understand the varied presentation of the diseases as well as the molecular mechanism of signal transduction of the receptor. [unreadable] [unreadable] Individuals with LHR carrying activating mutations develop familial male-limited precocious puberty (FMPP) and often have behavioral problems. This behavioral problem of FMPP patients may be related to the dysfunction of brain cells caused by the mutated receptor. Since the major growth period of mammalian brain occurs in the first trimester of pregnancy when hCG is high and LHR is expressed in brain cells, the hCG-LHR pathway has long been suspected to be involved in the development of the early brain. We attempted to assess the activity of LHR in brain cells using a rat neuronal cell line, PC12, which is bipotent and can differentiate into either neuronal or chromaffin cells upon exposure to neurotropins or differentiating inducing agents. PC12 cells expressing hLHR carrying the activating mutation Asp578His demonstrated neurite outgrowth in 10.8 ? 1.8 % of cells. This percent of cells with neurite outgrowth was significantly higher than that of cells transfected with vector or with wild type hLHR. Addition of hCG to the culture medium of stably transfected cells expressing wild type LHR caused a significant increase in the number of neurite-baring cells compared to that of no hCG treatment controls or vector expressing controls. The effect of hCG was dose and time-dependent. Differentiated cells appeared as early as 24 hrs after hCG administration and demonstrated profound morphological changes in 72 hours. The differentiated cells expressed early neuronal markers, neuronal specific tubillin III and neural filament 68, indicating that LHR activation either through a genetic mutation of the receptor itself or binding with its ligand induced the differentiation of PC12 cells toward neuronal cell type. Further studies showed that both p44/42 and p38 pathways were required for the neuronal differentiation of PC12 cells transfected with wild type hLHR and induced by hCG, while the SAPK/JNK MAPK and Akt pathways might not be involved. cAMP played an important role in transmitting signals from receptor activation to the signaling pathways involved. These results demonstrated a neural function of the hCG/LHR pathway. It also shows the neurotropic activity of hCG and its potential as a therapeutic agent for neurological disorders and acute injuries of the nervous system.[unreadable] [unreadable] Discovery of the presence of LHR with germline and somatic activating mutations in patients with testicular tumor raised the question of the tumorigenic potential of mutated LHR. Asp578His is a somatic mutation since hLHR carrying this mutation has only been found in testicular tumor tissues and has not been found in any patient with FMPP. On the other hand, Asp578Gly is the most common mutation detected in FMPP patients and can be transmitted through the germ-line. Animal studies have so far failed to establish lines of male or female transgenic founder mice carrying LHR with the Asp578His mutation. We speculate that in spite of the fact that Asp578Gly and Asp578His involve mutation of the same amino acid, the two mutant hLHRs have distinct biological effects and trigger expression of different sets of genes. To prove our hypothesis we compared the expression profile of MA10 cells transfected with mutated LHR carrying the germline activating mutation (Asp578Gly) with those expressing the somatic activating mutation (Asp578His). Results revealed different expression pattern consequential to the expression of the LHR with the Asp578Gly or the Asp578His mutation. We are in the process of delineating the biological pathways affected by these mutant LHRs.